Performance of amino-silylated fused-silica capillaries for the separation of enkephalin-related peptides by capillary zone electrophoresis and micellar electrokinetic chromatography.
Enkephalin-related peptides were separated at low pH in a capillary with covalently bonded aminopropyl groups. The peptides are electrostatically repelled from the capillary surface and much higher efficiencies and faster separations were achieved compared to separations using uncoated capillaries. At low pH the amino groups are protonated, which results in reversed electroosmosis. The influence of voltage and ionic strength on the mobility and the separation efficiency was studied. The repeatability of migration times within one day was very good with relative standard deviations of 0.3-0.7%. Increasing the pH decreased the electroosmosis, eventually turning towards the cathode in the pH range 5-6; the separation performance, however, was lower at higher pH. Neutral and anionic micellar agents were added to the background electrolyte at different concentrations; the enkephalins had weak association with the neutral micellar agents but were distributed to the anionic taurodeoxycholic acid (TDC) micelles, giving rise to changes in separation selectivities. Very high efficiencies were obtained for peptides with a low distribution to the TDC micelles, while the efficiencies were impaired for those with a strong association with the micelles, which may indicate a slow mass transfer in the association process.